1. Field of the Invention
The present invention relates to a solid polymer electrolyte fuel cell wherein a solid polymer membrane is used to obtain electric energy by an electrochemical reaction, and particularly to the structure of a flow path of air for use as an oxidizing agent gas.
2. Description of the related Art
FIG. 5 is an exploded perspective view showing the basic structure of a fuel cell as a minimum power generation unit of a solid polymer electrolyte fuel cell that is generally used. Catalyst layers 21 containing a precious metal, which is mainly platinum, are joined to the two surfaces of an electrolyte membrane 20 constituted of a solid polymer membrane to form a membrane-electrode assembly. Diffusion layers 22 are disposed on both other surfaces of the membrane-electrode assembly to play the roles of allowing either a fuel gas or an oxidizing agent gas to be passed to the catalyst layers 21 while performing a function of outward delivery of an electric current. They are interposed between gas-impermeable separators 23 to form a cell. In an example of the structure of FIG. 5, the surfaces of the separators 23 facing the respective diffusion layers 22 are formed in a ribbed structure wherein either a fuel gas or an oxidizing agent gas flows through grooves between ribs. A large number of fuel cells having the foregoing structure are laminated to form a fuel cell stack that is used as the battery body of a solid polymer electrolyte fuel cell.
The solid polymer membrane for use as the electrolyte membrane 20 is saturated with water to lower the resistivity of the membrane, whereby it can function as a proton-conductive electrolyte. In order to maintain the efficiency of power generation at a high level, therefore, the hydrous state of the membrane must be kept in a saturated state. In view of this, there is adopted a method wherein water is fed to a reactive gas to provide a high-humidity gas, which is then sent to a fuel cell to suppress evaporation of water from the membrane to thereby prevent the membrane from drying.
FIG. 6 is a basic system diagram showing examples of the reactive gas system and cooling water system of a conventional solid polymer electrolyte fuel cell. Air to be sent as an oxidizing agent gas to a fuel cell stack 1 is increased in pressure with an air feed blower 2 before being fed thereto. Air is admixed with moisture in a humidifier portion 4 attached to the stack 1, and then sent to the air electrode of every cell. In the humidifier portion 4, a method is used, for example, wherein air and cooling water respectively flow along the two surfaces of a thin water-permeable membrane to humidify air. Part of the oxygen in the air is reduced in the catalyst layers of a cell to form water. Air containing formed water and discharged from the stack 1 is sent to a condenser 6 to recover surplus water, and is then discharged out of the system. On the other hand, the fuel gas, which is supplied from a fuel feed source 3 such as a high-pressure hydrogen tank or a fuel reforming unit, is sent to the fuel electrode of every cell of the stack 1. The fuel gas may be passed or may not be passed through the humidifier portion 4 attached to the stack 1. In order to remove heat generated in the stack 1 by a battery reaction, there is provided a cooling water circulation system wherein a cooling unit 5 for cooling the cooling water by heat exchange and a pump 7 for sending the cooling water to the stack 1 are incorporated. Additionally stated, there is a case where a cooling water tank is provided though it is not included in the system of FIG. 6.
FIG. 7 is a basic system diagram of an example of another structure of the reactive gas system of a conventional solid polymer electrolyte fuel cell. In this structure, removal of heat generated in a fuel cell stack 1 is effected by air cooling with a cooling fan 9. Since this structure is not provided with a cooling water system, air increased in pressure and fed by an air feed blower 2 is discharged from the system without recovery of formed water.
As described above, in conventional solid polymer electrolyte fuel cells, there is adopted a method wherein heat generated in a stack 1 is removed by water cooling or air cooling to maintain a predetermined operating temperature.
In the method wherein cooling is effected with cooling water, however, there must be provided a cooling water system including a cooling unit 5 and a pump 7 incorporated thereinto. In the case where the cooling water system is used to conduct a long-term operation, the conductivity of cooling water must be maintained at or below a predetermined value to avoid a short-circuit between fuel cells. For this reason, a deionization unit (ion exchange resin), not shown in FIG. 6, must be incorporated into the system. This disadvantageously involves an increase in the number of units installed in addition to the fuel cell stack thereby increasing the weight and cost of the system.
On the other hand, the air-cooling method does not require a cooling water system as described above, but requires use of a large-capacity fan for introduction of a large amount of air because the cooling power of air is low as compared with water. Accordingly, there is a need for auxiliary machinery which is capable of enhancing the efficiency of power generation. Furthermore, since the cooling power provided by the air-cooling method is low, a difficulty is encountered in securing a high output density from the fuel stack making miniaturization of the stack difficult.